A hybrid drivetrain that acts in parallel with a serial arrangement of the components in the manner described above is possible in various embodiments.
For example, from DE 103 46 640 A1 a hybrid drivetrain of the above type is known, in which the electric machine is arranged coaxially with the input shaft of the drive transmission, the rotor of the electric machine is directly connected with the input shaft of the drive transmission in a rotationally fixed manner, and the drive transmission is made as an automatic planetary transmission.
DE 100 12 221 A1 describes a hybrid drive device with a main drivetrain and an auxiliary drivetrain. In the main drivetrain, which in the present case corresponds to the hybrid drivetrain, the electric machine concerned is arranged axis-parallel to the input shaft of the drive transmission and the rotor of the electric machine is in driving connection with the input shaft of the drive transmission via an input transmission stage EK, which is made as a spur gear pair with a high transmission ratio (iEK>1). The drive transmission is preferably an automatic planetary transmission. In such an arrangement of the electric machine the data presented below concerning the torque and the speed of the electric machine refer analogously to the reduced values at the output element of the input transmission stage or to the input shaft of the drive transmission.
In the present invention the drive transmission is preferably an automated change-under-load transmission such as an automatic planetary transmission, a dual-clutch transmission or a continuously variable transmission. However, the drive transmission can also be an automated change-speed transmission of countershaft design, in which shift processes are associated with a traction force interruption.
Furthermore, a starting element can be positioned immediately upstream from the drive transmission. Thus, in a manner known per se a hydraulic torque converter can be positioned upstream from an automatic transmission, which during normal driving operation, i.e. other than when starting off or maneuvering, is bridged by an engaged bridging clutch. Upstream from a continuously variable transmission and an automated change-speed transmission can, in each case, be positioned a starting clutch or a starting and shifting clutch made as a dry clutch or as a wet clutch, in particular a disk clutch, which is engaged during normal driving operation. The same applies to a dual clutch transmission, which—as is known—comprises two input shafts each respectively associated with a starting and shifting clutch. Alternatively, a starting element can also be integrated in the drive transmission, for example if a starting clutch associated with the input shaft is accommodated inside the transmission housing of the drive transmission, or if a frictional shift element of a drive transmission, made as an automatic transmission, which transmits load during starting, is used as the starting clutch.
A hybrid drivetrain of such type offers the possibility of operating a motor vehicle, as necessary, in a purely combustion-engine-powered driving mode, in a purely electric driving mode or in a combined driving mode. In the combustion-powered mode the separator clutch is engaged, the electric machine is switched to contribute no force and during traction operation the motor vehicle is driven solely by the drive torque produced by the combustion engine, and in thrust operation it is braked by the drag torque of the combustion engine then running in thrust operation, if necessary with further braking devices such as a service brake or a retarder in addition to the braking torque itself.
In the electric mode the separator clutch is disengaged, the combustion engine is switched off and, in traction operation, the motor vehicle is driven solely by the drive torque of the electric machine then being operated as a motor, whereas during thrust operation the vehicle is braked by the drag torque of the electric machine then operating as a generator, if necessary with further braking devices in addition to the braking torque.
In the combined driving mode the separator clutch is engaged. During traction operation the motor vehicle is driven by the sum of the drive torques of the combustion engine and the electric machine, whereas in thrust operation it is braked by the sum of the drag torques of the combustion engine and the electric machine then operating as a generator, if necessary with further braking devices in addition to the braking torque.
Apart from the hybrid driving modes considered above, the combustion engine and the electric machine can if necessary also be operated with different force flow directions, whereby the torques produced partially cancel out. Thus, in certain traction operation phases it can be appropriate to operate the electric machine as a generator in opposition to the action of the drive torque from the combustion engine, for example in order to charge an empty electric energy accumulator, or in order to operate the combustion engine at an optimum operating point. Likewise, in certain thrust operation phases it can be appropriate to operate the electric machine as a motor in opposition to the drag torque of the combustion engine, for example to keep the combustion engine above a critical rotational speed limit.
Owing to the variety and different operating properties of the possible embodiments of such a hybrid drivetrain, known control methods mostly assume at least a certain power of the electric machine and/or a certain structure of the drive transmission. Furthermore, the known control methods are often limited to the solution of partial problems that occur during the control of the specific hybrid drivetrain concerned.
DE 43 24 010 C2 discloses a method for controlling a corresponding hybrid drivetrain which starts with a structure of the drive transmission in the form of an automatic planetary transmission with a hydraulic torque converter immediately upstream from it. In this known method, during pure electric driving operation the electric machine is controlled in such a manner that the torque characteristic of a combustion engine is simulated. Furthermore, the known method provides that when the motor vehicle concerned is operated in thrust mode, the braking torque (drag torque) of the combustion engine is supplemented or replaced by a braking torque produced by operating the electric machine as a generator.
A similar method for controlling a corresponding hybrid drivetrain is described in DE 101 50 990 A1. This known method also starts with a structure of the drive transmission in the form of an automatic planetary transmission, but this time with no hydraulic torque converter upstream from it. In this method too, during pure electric driving operation the electric machine is controlled in such manner that the operating behavior of the combustion engine is imitated. When electric-powered driving (with the combustion engine inactive and the separator clutch disengaged) is changed to combined driving operation or to combustion-engine-powered driving operation, the combustion engine is started by engaging the separator clutch, this being intended to ensure a soft transition, i.e. one with very little jerkiness, to the power of the combustion engine. However, specifically how the separator clutch and the electric machine should be controlled in order to achieve this outcome is not explained in DE 101 50 990 A1.
Another method for controlling a corresponding hybrid drivetrain is known from DE 10 2005 007 966 A1, which starts with a structure of the drive transmission as a change-speed transmission, i.e. an automated change-speed transmission with gearshift-related traction force interruption. In this known method the electric machine, clearly one with relatively low power, is used primarily as a starter motor for the combustion engine, and an electric generator with an auxiliary drive for synchronizing the respective target gear of the drive transmission during gearshift operations. In addition the electric machine is intended for use in other auxiliary functions such as preventing or minimizing rolling downhill, performing a crawling function and for active damping of rotation oscillations. In contrast, no provision is made for using the electric machine as a main drive motor so DE 10 2005 007 966 A1 does not relate to any purpose beyond the auxiliary functions mentioned.
During the operation of a hybrid drivetrain of the structural form considered here, the general problem arises, when changing from the combustion-engine-powered mode or the combined mode to the electric mode, of how to effect the change in the smoothest possible way, i.e. with little or no jerkiness, and thus in a manner that is comfortable for the occupants of the vehicle and which reduces the wear of the hybrid drivetrain's components.
As may be generally appreciated, a smooth change from the combustion-powered driving mode in which the combustion engine is operating in traction, the separator clutch is engaged and the electric machine contributes no force, or from the combined driving mode in which the combustion engine is operating in traction, the separator clutch is engaged and the electric machine is operating as a motor, to the electric driving mode in which the combustion engine is switched off, the separator clutch is disengaged and the electric machine is operating as a motor, is achieved by first reducing the torque of the combustion engine down to zero by means of the engine control system and at the same time correspondingly increasing the torque of the electric machine in its motor operation.
When the torque of the combustion engine has been reduced to zero, the separator clutch is disengaged and the combustion engine is then switched off.
Since the torque of the electric machine is increased to the same extent as the torque of the combustion engine is reduced, and the separator clutch is only disengaged once the torque of the combustion engine has become zero, a change carried out in this way from the combustion-powered or the combined driving mode to the electric-powered mode takes place substantially free from jerkiness and essentially without any change of the resulting drive torque on the input shaft of the drive transmission.
Analogously to this, during thrust operation of the motor vehicle a smooth change from the combustion-powered mode in which the combustion engine is operating in thrust, the separator clutch is engaged and the electric machine contributes no force, and from the combined driving mode in which the combustion engine is operating in thrust, the separator clutch is engaged and the electric machine is operating as a generator, to the electric driving mode in which the combustion engine is switched off, the separator clutch is disengaged and the electric machine is operating as a generator, is achieved by first reducing the drag torque of the combustion engine to zero and at the same time reducing the torque of the electric machine correspondingly, i.e. increasing the drag torque produced by the electric machine when it is operating as a generator to the same extent. When the drag torque of the combustion engine has been reduced to zero, the separator clutch is disengaged and the combustion engine is then switched off.
However, the disadvantage of this last-mentioned procedure during thrust operation of the motor vehicle is that the combustion engine, running in thrust operation and switched appropriately for thrust, has to be ignited in order to reduce its drag torque. This means that the supply of fuel and if needs be the ignition must be switched on in order to bring about the reduction of the drag torque and to be able to control the timing of the drag torque reduction by means of the engine control system in the desired manner. This on the one hand entails relatively large and time-consuming control complexity, and also results in unnecessary fuel consumption and corresponding exhaust gas and noise emissions.